A. Field of Invention
The present invention relates to lighting fixtures, and in particular, to monitoring the performance of and allowing for compensation for lumen depreciation or other losses of light during operation of lighting fixtures. In one example of performance, if monitored actual lumen output drops below a preset threshold level, appropriate action can be taken, such as servicing or automatic compensation.
B. Problems in the Art
Almost any light source has a life span. One problem with certain types of light sources is that during their normal operating life span, the amount of light they produce diminishes; they lose lumen output (or flux) over operating time.
One example is HID lamps, the type frequently used in wide area lighting systems such as sports lighting. A Model MH 1500W/HBU metal halide 1500 watt lamp commercially available from Venture Lighting International, Solon, Ohio USA is an example. These lamps, as well as relatively all HID lamps used in the industry, exhibit the well-known lamp lumen depreciation (LLD) function (see, for example, Venture Lamp Catalog, 2004 edition, Venture Lighting International, page 93 Lamp Technical Info, which is incorporated by reference herein and which includes a lumen depreciation or lumen maintenance curve, as published by that lamp manufacturer). LLD, in these types of lamps, results in a relatively substantial decrease in lumen output after a first operating period (usually the first 50 to 200 hours of operation), then a less substantial rate in the next 1000 hours or so. Thereafter, lumen decrease continues, but at a substantially slower rate. A typical lumen maintenance curve for a variety of groups of field and laboratory tested 1500 watt metal halide HID lamps is shown in FIG. 8. This testing was conducted by technical personnel of Musco Lighting (Musco) of Oskaloosa, Iowa, USA. It illustrates the relatively consistent decrease in lumen or light output or quantity over cumulative operating hours, including a relatively quicker rate of drop early and slower rate of drop later. But it also shows there is variability between groups. There can also be variability between individual lamps, including between lamps of the same manufacturer model and specifications.
Lamp manufacturers usually attempt to provide customers with information that characterizes LLD for the particular lamp. Many times the manufacturer gives a rather specific indication of expected magnitude of the rather dramatic lumen drop that occurs during the first operating period. They typically consider the lamp lumen output “stabilized” after this initial substantial lumen decrease; many times stated as or called “100-hour burn-in”. Some manufacturers also give what is called a lumen depreciation factor (LDF) (sometimes, referred to as lamp maintenance factor) or other specification that generally characterizes LLD over the entire predicted normal operating life of the lamp. For example, a typical LDF is 0.75. A more generous factor is 0.82. Both indicate in a very general way that the lamp will tend to decrease in lumen output over the lamp's life, down to 75 percent of its original output (for LDF of 0.75) or 82 percent for LDF of 0.82 (see, e.g., FIG. 8). LDF is a dimensionless value between 0 and 1.0 which is related to the proportion of light output of a lamp produced after a set time of operation to that provided when new. There are generally two ways to obtain LDF. One is from the lamp manufacturer's LLD curve or chart. The other is by dividing maintained lumens of the lamp by initial lumens. However, maintained lumens are generally rated at 40 percent of the lamp's life, so at best it provides a relative indication of the predicted lumen depreciation. The amount of depreciation in the remaining 60% of the lamp's life would need to be estimated.
Another definition of LDF is the fractional remainder of lamp lumens, at rated operating conditions, due to lamp degradation. The greater, i.e. closer to 1.0, the LDF means a lamp will remain brighter longer.
Lamp life is defined by lamp manufacturers as the length of time in hours when 50 percent of the lamps in a group are expected to fail. In sports lighting, for example, lamps are usually elevated on poles or superstructure. To replace a lamp usually requires a lift or crane. Thus, to prevent the cost, resources, and inconvenience of spot replacement of individual lamps ad hoc as they fail, and to promote energy efficiency, the IESNA (Illuminating Engineering Society of North America) promotes group lamp replacement at 70% of rated lamp life. For a typical 3000 hour rated lamp, this would be at 2100 hours of cumulative operating time. A worker need only be lifted one time to service all lamps on each pole.
However, lamp performance has improved in recent years, which has resulted in considerably fewer lamp failures up to the rated life of the lamp. Consequently, most customers choose not to replace the lamps at 70% of life, but instead continue to operate them until the quantity of lamp failures causes significant loss in light produced on the target area.
Thus, due to this extended life, prediction of LLD becomes more critical to avoid insufficient light on the target area prior to lamp replacement. But, at best, LDF is quite arbitrary because it is a rough generalization and each lamp differs in its operating characteristics over time.
While published LLD essentially allows the manufacturer to take the position that they have warned the customer of the expected initial light loss from its lamp until it is “stabilized”, the customer is left to deal with it. And, if the manufacturer gives an LDF to the customer, it is an even more general estimation of expected light loss after the initial period, and the customer, again, is left to deal with it.
Giving published predicted LLD and/or LDF can attempt to absolve the lamp manufacturer of responsibility if lumen output of the lamp does not meet or exceed their predictions. Furthermore, because LLD is so difficult to predict after the lamp is stabilized, and because it is relatively gradual, lamp manufacturers tend to disclaim any liability after lamp stabilization. Some lamp manufacturers voluntarily replace a lamp if it fails or loses substantially more light than the manufacturer's predicted amount until it “stabilizes” (e.g. the first 100 hours or so of operation). The customer is truly left to deal with the lamp after stabilization, which is after the first 100 hours of operation.
Historically, one approach to deal with this lamp characteristic is for the lighting system designer (usually not the lamp manufacturer) to take initial lumen depreciation into account when designing the lighting system. Essentially, the predicted light output is used in the calculation to determine the quantity of fixtures needed to meet the desired light level. This approach compensates roughly for initial lumen depreciation, but over operating time, light output will continue to drop. Many times it drops to an unacceptable level. The customer must either live with this or replace the lamp relatively early in the lamp's life, often as soon as one-third of the useful life. A typical HID lamp life is at least thousands of hours, generally in the range of 3,000 or more for 1500 watt lamps. Thus, the lamp manufacturer generally will not monetarily compensate for or replace the lamp because it is well after the initial 100 hour period. Also, lumen depreciation to unacceptable levels at one-third of normal lamp life (e.g. at roughly 1000 hours of operation for a 3000 rated life lamp) either forces the customer of the lighting system to spend the cost of replacement or to suffer unacceptable light levels for a potentially lengthy time—e.g. more than one-half of anticipated normal life. Again, the problem falls on the customer's shoulders.
Most customers of sports lighting (e.g. public institutions such as school districts, park and recreation departments, municipalities, and the like, or their private sector analogs like private schools, sports clubs, and the like) are not lighting experts. Therefore, they tend to let LLD go instead of dealing with it. They may not even appreciate it. Lamps tend to keep operating at the end of useful or rated life even if they are no longer useful. They appear “on” but do not supply a useful amount of light. This can have potentially negative results. Inadequate lighting levels can make it both more difficult, and less enjoyable, to play a sport or to watch it. There can even be concerns of injury to sports players. Organizations such as IESNA and others have developed sports field illumination intensity and uniformity minimums for different sports for these purposes. Many customers try to insure against these types of problems by specifying that such standards be met by the entity installing the sports lighting system.
Problems with this approach can include the following. The lighting system installer typically must rely on the lamp manufacturer LLD or LDF specifications. However, as discussed above, they are not normally precise enough. Also, no two lamps are identical on these points. One lamp of identical nominal operating wattage and other structure can vary dramatically in initial lumen drop as well as lumen drop after lamp stabilization. Relying on lamp manufacturer specifications also assumes the lamp will perform as predicted despite other factors that can affect it. Therefore, using the lamp manufacturer's specifications is merely an educated guess for individual lamp performance.
Therefore presently many owners or operators of such lighting systems periodically, without reference to actual need, change out lamps, clean lenses and reflective surfaces, and perform other maintenance tasks on each fixture periodically to try to meet IESNA light level guidelines. But this is costly and cumbersome in terms of time and labor resources. It also is done regardless of actual need. The tasks may be done too late to avoid violation of the guidelines, or they may be done too early, in terms that the work is not needed. Therefore, there is room for improvement in this area.
One relatively recent approach to addressing LLD is the commercially available SMART LAMP® system from Musco Lighting, Oskaloosa, Iowa USA. Circuitry automatically adds operating power to each lamp at pre-set times over operating life of the lamp to compensate for LLD. The manufacturer's LLD and LDF information or information obtained from extensive testing is used to pre-set the times and amount of increased wattage to keep lamp light output more constant over operating life (e.g. +/−10% of a predetermined level, or even better). Details about SMART LAMP®-type systems can be found in U.S. Published Application 2005/0184681A1 issued as U.S. Pat. No. 7,176,635 on Feb. 13, 2007, which is incorporated by reference in its entirety herein. It allows an end-user customer to have a lighting System that ensures near constant light output over the normal operating life of such lamps. This avoids arbitrarily replacing lamps well prior to end of their normal life span or ignoring LLD and leaving lamps in place beyond their useful life (i.e. where the value and/or amount of the light produced does not warrant the energy consumed). It also allows a lighting system installer or designer to be able to take over responsibility for maintaining a relatively consistent light output for the lamps over their entire normal operating life. It allows such responsibility for the entire operating life of the lighting system.
A specific sports lighting example further illustrates the issues in the state of the art. Lamps of typical sports lighting type (mentioned above) have nominal operating lives of thousands of hours (e.g. 3000 hours). One present approach is to replace all lamps at 1200 or 2100 hours of operation because of lumen loss (assumes total LDF of 0.8 and 0.7 respectively). A SMART LAMP® system allows lamps to be run at least their nominal life span, and substantially more (e.g. three or more times longer in some cases). But further, typical sports lighting systems (poles, light fixtures, wiring and circuits) can last decades. The SMART LAMP® system can be installed one time. It can therefore be used to maintain light output of the lamps, and decrease replacement of lamps multiple times over those many years of normal operating life of such systems. It therefore allows a lighting system installer or designer to take responsibility for light output of the system for its entire normal life span. A sports lighting system typically is operated 3-6 hours/day. This translates into one to two thousand hours per year. Over twenty years, this can mean from approximately six to twelve (or more) sets of lamps based on 3000 hours normal life span.
However, the foregoing has several potential issues. The approach generally only compensates for predicted loss in light output from the lamp and some allowance for other factors. It does not address actual total light output loss from the lighting fixture or fixtures. There is also a need in the art for compensation of other types of light loss from lighting systems. Lumen depreciation can include not only LLD (lamp lumen depreciation) but lumen loss from dirt or debris (i.e. LDD or luminaire dirt depreciation that accumulates on the glass lens of the lighting fixtures or on optical surfaces), and other factors known in the art.
LDD is a common loss in enclosed fixtures. It can be caused by atmospheric dirt or other substance(s) accumulation on the outer lens surface. It can also be caused by dirt or other substance(s) or deposit(s) on interior fixture components (e.g. atmospheric dirt or dust which enters the fixture or a film from chemical out gassing). Out gassing is the result of vapor release as foreign objects and components of chemical composition are heated up. There can be other substances that absorb, obstruct, or otherwise disrupt light from leaving the fixture in a useful manner. IESNA refers to these types of lumen loss as lumen dirt depreciation or LDD. Manufactures often do provide an LDD factor, similar to the LLD, to account for such loss in output. A typical factor may be 0.93, which can be combined with the LLD to represent an approximation of total losses. For example, an LLD of 0.75 with an LDD of 0.93 will result in total loss factor of 0.7 by known in the art calculations. However, the amount of LDD losses may be significantly more than estimated if the fixture design does not prevent out gassing or if care is not used to prevent foreign bodies or materials in the internal fixture assembly during manufacture, assembly, installation, or operation.
There are other potential causes for light loss from a lighting fixture. Some are called “non-recoverable” because preventive maintenance does not generally effect the extent of such loses and the losses are not recovered by maintenance. Some examples are ballast factor, supply voltage variation, and lamp tilt factor. The present invention pertains primarily to what are sometimes called “recoverable” factors; specifically LLD, LDD, and lamp burnout.
Thus, there are many factors that contribute to loss in light output from the fixtures. Since these factors vary from fixture to fixture, it is difficult to accurately predict them. A need therefore exists to assure the customer that its lighting system will achieve a desired light level and hold that level relatively constant over time. A better solution is not indicated if based on just a lamp manufacturer's generalized, rough estimate of LLD and LDD.